Tubular heating unit



c. T. JOHNSON-VEA ET AL 1,754,003

April 8, 1930.

TUBULAR HEATING UNIT Filed Aug. 14 1925 l1; VEN TORS A TTORNEYI-S Patented A r; 8, i930 UNITED. STATES PATENT" ounce f CHARLES T. JOHNSON-TEA AND WILLIAM A. BRA'UN, OlF DOVER, OHIO, ASSIGNOBS TO DOVER MANUFACTURING COMPANY, OF DOV ER, OHIO TUBULAR ,HEATING um Application filed August 14,

Our invention relates to improvements in electrical heating units and methods of making the same, with particular reference to the manufacture of tubular heating units in which ahelical coil or convoluted resistance member is embedded in granular material associated with a suitable binder.

The object of this invention is to provide an improved tubularheating'unit in which the number of voids are reduced to a minimum and the contacts of the grains of embedding material with each other and with the resistance element and the tube are substantially at maximum,-the resistance element, em-- bedding material and tube being formed as nearly as possible into one non-hygroscopic solid, highly resistant to mechanical abuse and capable of being used with uniform efliciency for long periods of time.

More specifically stated, the object of this invention is to provide an improved tubular heating uriit in which a resistance element is embedded in a mass of granular material having high dielectric strength, the voids between the grains being filled, in'part, by a more finely divided granular bonding material and the smaller voids thus created being, in turn, filled with boracic acid applied in the form of a dried powder which, when heated,

melts and oxidizes and constitutes a flux for the bonding material, whereby the entire mass becomes cohesive and solid, being bound together and converted from a mechanical mixture in which the parts are consolidated by vibration into a mechanical combination of unitary character. A further object is to provide anv improved method of manufacturing heating units having the above mentioned characteristics, whereby such manufacture may be expedited and the cost thereof reduced to a minimum.

In the drawings: I A Figure 1 is an elevational view illustrating the improved prooe'ss, showing a partially completed heating unit and an associated vibrator employed for compacting the embedding material.

Figure 2 is a plan view of the centering cap 4 binding post and refor holding the upper during the operation sistance element in place 1926. Serial No. 129,186.

of pouring and compacting the embedding material.

Figure 3 is a side elevation of the completed heating unit. J

Like parts are identified by the same ref erence characters throughout the several views. I

In the practice'of the improved process, a metal tube 10 is fitted with a'cap 11 having a plug 12 adapted to enter the tube in closely fitting relation to its walls andhaving threaded apertures to receive the screws '13, which pass through apertures formed in the tube 10 by inner and outer nuts 17 and 18 respectively.

. A convoluted resistance element 20 has terminals 21 axially secured respectivelyto the binding post 16 and to binding post 22 at the opposite end of the tube 10. The resistanceelement preferably has the form of a helical coil of the required cross section and lengthto develop the desired number of heat units when an electrical current of predetermined volume and pressure is passed through it.

In the manufacture of the unit,.the binding post 22 is centered and held 1n position with the axis of the coil 20 substantially coinciding with the axis of the tube, this being accomplished by meansof the temporary cap ring 25, (Figure 2') this ring having a central annulus 26 supported by the inwardl pro-r jecting web 27. The aperture 28 in tile annulus is threaded and is adapted to receive the binding post 22 as, clearly illustrated in Figure 1, whereby the binding post may be held in the proper position while the embedding material is being poured into the tube through the 'arcuate opening 29 in the cap ring 25.

The tube is then held in an uprightp-osi tion, with the cap 11 at the bottom, and connected with a motor-actuated vibrator. In

the drawings I have illustrated an-air engine 30 of an ordinary type provided with clamp ing members 31 to engage the tube under pressure exerted by the clamping screw 32. The engineis supplied with air through the flexible hose 33. The construction of such air engines being well understood, it is sufficient to say that when set in operation, its piston will reciprocate the clamping members 31 with great rapidity, thereby causing the tube 10 to vibrate rapidly in a horizontal plane. The clamping members 31 are preferably engaged with the tube 10 near its lower end whereby the upper end of the tube is permitted to vibrate freely with a slightly arcuate movement due to the yielding of the parts. While the tube is being thus vibrated, the embedding material, hereinafter described, is poured into the upper end of the tube through the arcuate opening 29. This embedding material is preferably composed of finely divided granular fused aluminum.

oxide mixed with a suitable binder such as clay, (the form of clay known as Klingenberg clay being preferred), and a quantity of dry powdered boracic acid. The rapid vibration of the tube causes the granular particles to settle under the influence of gravity, whereby intimate contacts are secured between each particle and the adjacent particles and between particles and the resistance element and tube. The particles or granules are caused to readjust andjmpact upon each other in a manner to reduce the. voids to a minimum. In general, thegranular aluminum oxide grains will be larger than those of which the binder is composed, although the latter may contain some large grains and some which are exceedingly small and have the characteristics of particles ofdust. The powdered boracic acid is composed of still smaller grains and much of it has the characteristic of flour. Some of it is so fine as to be substantially impalpable. Therefore, the vibration of the particles causes the bonding particles to settle into and partially fill the voids between the granular aluminum oxide and these, in turn, present smaller voids "which are filled by the particles of boracic acid, thereby producing an exceedingly thorough mixture and distribution of the boracic acid throughout the mass, but also producing an embedding mass. which has substantially the dielectric strength of the fused aluminum oxide, since there remains no direct flux paths from the resistance element and the walls of the container which are not intercepted by\ the aluminum oxide grains.

Such paths as mightbe established through the bonding material and boraciclacid are so increased in length as to afford the requisite dielectric strength even though the resistance may be less than that of the aluminum oxide forta flux path of any given length and cross see 1011.

The rapid vibration also causes portions of the mixture to work into the coil and into the interstices and completely fill the same to the exclusion 'of air therefrom, thereby developing an exceedingly dense pack in the portion of the tube occupied by the resistance member or coil. The particles particularly tend to develop wedging pressures along the oblique metal surfaces. The operation of pouring the embedding material into the tube may be continued until the tube has been filled with compacted material to the upper end of the coil or the lower end of the binding post 22. If desired, water may be added to the mixture and the mixtureconverted into a paste-like mass before pouring it into the tube, whereuponthe particles may be com.- pacted by gravity and the interstices also filled, in part, by flotation. Where water is used, the surplus liquid may be allowed to flow away from the top of the tube while the particles are being compacted. If desired, the vibration of the tube may be stopped and the tube held at rest for a short interval before releasing it from the clamp and pouring off the liquid from the upper end, therebyallowing the compactedmaterial to set sufliciently to retain its position. The boracic acid and the bonding material tends to become adhesive and coherent under such conditions.

Either before or after the tube has been released from the vibrator clamp, the binding post may be connected in an electrical ,circuit and current passed through the heating element or coil 20 to dry and bake the embedded material into a solid mass. The boor impair the resistance element and is also below the fusing point of the aluminum oxide grains. The boracic acid, when introduced into the mixture, is in the state known as H BO When the unit is heated, the boracic acid constituent is I melted, forming boric 'cxi'dc (B 03) and this coats theresistance wire and the inner surfaces of the tube. Itmixes with .420

the binding material and fills'the interstices between the granules, thereby forming an exceedingly dense mass which becomes largely impervious to water and not only prevents oxidation, but also prevents the loss of dielectric strength which would otherwise occur through moisture absorption.

The specific composition of embedding material herein described is not essential to the method herein described, it being obvious that any granular embedding material may be similarly vibrated and compacted within the tube and about the resistance element preparatory to allowing it to harden or preparatory to' baking it, if it contains boracic acid or an equivalent finely divided .flux. Specific proportions of the highly dielectric material, the bonding material, and the boracic acid are not essential, but it will ordinarily be found desirable to employ a mixture in which the fused aluminum oxide comprises from 7 0 to 90 percent of the total volume. Substantially equal parts in volume of bond ing material and boracic acid may be employed. These proportions relate to the materials as mixed preparatory to the heating or melting stage of the process.

After the drying, hardeningand baking operations have been completed, the temporarycap ring mayberemovedand a perma nent cap similar to the cap 11 may be applied to the binding post 22, thereby completing the unit.

\Ve are aware of the fact that in the molders art it has for many years beemcommon practice to vibrate m'olds in order to compact the grains of molding sand upon each other and upon the pattern, and therefore no claim is made herein for the mere compacting of substantially dry granular material in this manner. But we believe we are the first to vibrate a container while it is being filled with a mixture of granular material composed of dielectric and bonding materials in grains or particles of different sizes and combined with a chemical agent, whereby the mass may be converted into a non-hygroscopic unit solidified by drying and baking it after the granular particles have been compacted by gravitation, or by gravitation and partial flotation, to form a mass of considerably greater density and freedom from voids than is possible to produce by vibration of a homogeneous material. By the process herein described, the embedding material bakes into a stone-like mass, substantially impervious to moisture and the particles are-incapable of shifting upon each other while confined inthe tube even though the tube may be subjected to blows or bending stresses.

We claim: V f

1. The method of making tubular heating units consisting in suspending a resistance element axially within a tube, providing the same with suitable terminals such as binding posts, closing one end of the tube and supporting it in an upright position with the other end uppermost and open, vibrating the tube in a horizontal plane and simultaneously filling it with an embedding material com posed of a mixture of granular materials of different sizes and having different melting points whereby the material is capable of being converted into a unitary stone-like nonhygroscopic mass when heated without melting the more refractory portion of said material, and then heating the tube and its contents to melt the less refractory portions of said material and subsequently capping the uncapped end of the tube with the terminals exposed.

2. The method of making encased tubular heating units, consisting in preparing a mixture of a quantity of granular dielectric ma terial, a more finely divided quantity of bonding material, and a still more finely divided quantity of boracic acid, compacting said material about an electric resistance element by vibration in an enclosing casing containing such unit in a manner to cause the bonding material to fill voids between grains of dielectric material and'to further cause the boracic acid particles to fill the smaller voids, and then raising the temperature of the unit sufficiently to cause the mixture to bake into a stone-like non-hygroscopic mass under the influence of the boracic acid..

3. The method of making encased tubular heating units, consisting in. preparing a-mixture of a quantity of granular dielectric ma terial, a more finely divided quantity of bonding material, and a still more finely divided quantity of boracic acid with sufficient water to form a flowing pasty mass, compacting said material about an electric resistance element by vibration in an enclosing casing containing such unit, and then raising the tempera ture of the unit sufficiently to cause the mix- 7 ture to bake into a stone-like non-hygroscopic mass under the influence of the boracic acid.

4. The method of making tubular heating elements, consisting in supporting a resistance element axially within a casing in spaced relation to the walls thereof, filling the space with a mixture of highly refractory granular dielectric material, a void filling bonding material, and a more finely divided material fusible at a lower temperature than that of the resistance element, vibrating the casing to cause the smaller particles to settle into the voids between the larger particles, and then heating the unit to a temperature where the less refractory material will soften and unite the other materials.

CHAS. T. JOHNSON-VEA.

WILLIAM A. BRAUN. 

